An electric motor is an electromechanical converter ( electrical machine ) that converts electrical power into mechanical power . In conventional electric motors , current-carrying conductor coils generate magnetic fields whose mutual forces of attraction and repulsion are converted into motion . This makes the electric motor the counterpart to the generator , which has a very similar structure and converts movement power into electrical power. Electric motors usually generate rotating movements , but they can also be built for translational movements ( linear drive ). Electric motors are used to drive a wide variety of equipment, work machines and vehicles.
In 1820 the Danish physicist and philosopher Hans Christian Ørsted discovered the magnetic effect of electric current, a fundamental phenomenon of electromagnetism . A year later, Michael Faraday published his work on "electromagnetic rotation". He constructed a device in which an electrical conductor rotated around a fixed magnet and, in a counter-experiment, a movable magnet rotated around a fixed conductor. In 1822 Peter Barlow developed the Barlow wheel named after him . The British scientist William Sturgeon invented another engine precursor in 1832. On the European continent, Ányos Jedlik (1827) and Hermann Jacobi worked on the further development of the direct current electric motor. Jacobi developed the first practical electric motor in Potsdam as early as 1834 and in 1838 equipped a 12-person boat in Saint Petersburg with the 220 watt motor he had developed, which was also the first practical use of an electric motor. The American blacksmith Thomas Davenport also developed a commutator motor in Vermont . He was granted a patent for his design on February 25, 1837.
Thus around 1837/1838 the basis for an electric motor drive was known and developed into a work machine suitable for use. Werner von Siemens patented his dynamo machine in 1866 . It enabled the generation of electrical energy on a large scale for the first time. This helped the electric motor achieve a breakthrough for practical widespread use. In addition, there were also some technical developments of different types of electric motors at that time, but in the end they were of no importance. These include the Egger electric motor , which is constructed similarly to a steam engine, and the electric motorcycle by Johann Kravogl .
From around 1880, electrical networks and power plants were built in many countries. In Germany, for example, Emil Rathenau was a pioneer with his Allgemeine Electricitäts-Gesellschaft and in America Thomas Alva Edison . With the large-scale provision of electrical energy, the electric motor then spread quickly. Together with the chemical industry , this electrification was the most important characteristic of the second industrial revolution . The public horse-drawn trams were replaced by electric trams , and electric motors were now used instead of the steam engine to drive a wide variety of work machines.
Basic principle / functionality
The rotary motion of an electric motor is based on the attraction and repulsion forces that several magnetic fields exert on each other ( Lorentz force ). In the usual electric motor there is a fixed outer part and an inner part rotating in it. Either one of them has permanent magnets and the other has electrical coils, or both components have coils. Every coil through which current flows generates a magnetic field, the direction of which (north pole / south pole) depends on the direction of the current - if the current flows in the opposite direction through the coil, the magnetic field is also reversed. By repeatedly reversing the polarity of the coils during one revolution, continuous rotation of the inner part is achieved.
The fixed, magnetically acting part of an electric motor is called a stator. In the case of electric motors, the stator is usually on the outside and is connected to the housing; If the stator is on the inside, the motor is called an " external rotor ".
The moving (mostly: rotating), magnetically acting part of an electric motor that turns the motor axis. It consists of the axle, the armature and a coil, if the armature is not a permanent magnet.
Iron core of the rotor around which the rotor coil (s) is / are wound.
Shoe-shaped bulge of the iron of a magnetic core, which is intended to guide / concentrate the magnetic field at this point.
A disk with electrical connections that are segments of the disk; the disc rotates with the rotor shaft. The coils are connected to the connections; the commutator disc reverses the polarity of the coils during one revolution. The exact functionality is explained in the following section.
DC motor (commutator motor)
The (fixed) stator in a DC motor can be a permanent magnet with pole shoes , but external excitation via an excitation coil is also possible instead of the permanent magnet. In the case of an AC commutator motor or universal motor, however, there is always an excitation coil in the stator. If current is passed through this coil, the excitation field (magnetic field) builds up ( Ørsted principle).
Inside the stator is a rotor , which in most cases consists of a coil with an iron core (the so-called armature ), which is rotatably mounted in the magnetic field between the pole pieces of the stator.
The power supply for the armature takes place via a segmented commutator and sliding contacts ( carbon brushes ). If you send current through the rotor, a magnetic field is created here as well, which now interacts with the magnetic field of the stator. It thus rotates around its own axis and always switches the appropriate windings into the current path via the commutator that rotates with it and can thus convert electrical work into mechanical work.
If such a motor did not have a commutator, the armature would rotate until the rotor magnetic field is rectified to the stator field. So that it does not stop at this “dead point”, the current in the armature coils is switched with the help of the commutator (also called commutator or collector) with each new segment. The commutator consists of metal segments that form a cylindrical or circular surface interrupted by narrow strips of non-conductive material (plastic, air). The armature windings are connected to the segments. On the commutator, pressed by springs, there are usually two carbon brushes that supply the current. With each rotation of the rotor, the direction of current through the armature windings is changed and those conductors whose current flow is directed in such a way that a torque is generated enter the magnetic field of the stator.
The magnetic field in the rotor is - relative to the stator - fixed; the iron core of the rotating armature must therefore consist of a stack of sheets to avoid eddy currents.
AC motors can also be built according to this principle if the excitation field changes polarity with the alternating current ( universal motor ). Then the stator must also consist of a laminated core.
AC and three-phase motors
With alternating current , a commutator can be dispensed with if the number of revolutions is in the rhythm of the alternating current; the then rotating magnetic field of the rotor is then generated:
- through currents induced by the excitation field in a short-circuit winding ( asynchronous motor )
- by magnetizing an iron core with poles ( reluctance motor , stepper motor )
- by permanent magnets (stepper motor, electronically commutated direct current motor, synchronous motor)
- by an electrically excited rotor (see e.g. pole wheel or excitation systems for synchronous machines )
Such motors therefore have little or no starting torque. You need start-up assistance, but you can also start yourself with alternating current with more than one phase:
- Three-phase motors are operated with three-phase current, which consists of three alternating voltages which are phase-shifted by 120 ° and thus generate a rotating field
- Capacitor and shaded pole motors generate an auxiliary phase (a rotating field) for starting from a single-phase alternating current.
- Stepper motors and reluctance motors are operated with variable-frequency alternating current and / or with several phases, so that they stay “in step” and no step losses occur.
- Synchronous motors require starting assistance or rock / swing themselves "in step".
Types of electric motors
Rotating field and traveling field machines
- Three-phase motor
- Linear motor
- AC motor
- Transverse flux machine
Commutator or commutator machine
- DC motor
- Universal motor (for direct and alternating current)
- Repulsion motor
There are some types of electric motors that are of no economic importance today.
Instead of magnetic fields , electrostatic motors use electric fields that are generated by charges. Due to the high voltages required and a lower degree of efficiency, these motors are only relevant for small forces and scales.
Electric motors are used both unregulated and regulated. In simple cases, uncontrolled three-phase motors with star-delta switching are used. However, these are only suitable for solving primitive drive tasks. In most cases in today's practice, there are more demanding drive problems, so that the electric motors must be regulated by a closed-loop control. If the power that is required is greater, power electronic actuators must be interposed between the control and the electric motor. If the control and the electric motor come together and together they form a functional unit, one speaks of the " electric drive ". An electric motor is therefore not tied to a regulation per se; in many practical cases, however, it is precisely their interaction that has proven to be useful.
In the past, electric motors initially found practical use as a drive for trams and a little later as a universal drive to replace steam engines in factories, and for this purpose they were used via belt drives to drive mechanical looms and the like. With the introduction of assembly lines in industry, electric motors became the driving force behind entire branches of industry.
In the traffic and mobility sector, electric motors were first used in electric locomotives and electric railways, later in electric carts and forklifts . With the further development of rechargeable batteries, electric cars are being built with ever greater range and will be seen as an alternative to the internal combustion engine in the future due to the high efficiency of the electric drive . Developments in power electronics brought a further boost to applications - from then on, the maintenance-free, inexpensive asynchronous motors could also be used for variable-speed drives.
Today electric motors are used in large numbers in machines, automats, robots, toys, household appliances, electronic devices (e.g. video recorders, hard drives, CD players), in fans, lawn mowers, cranes , etc. The great importance of the electric motor for today's modern industrial society is also reflected in the energy consumption : Electric motors have a share of over 50 percent of electricity consumption in Germany.
Electric motors in mobile applications
Electric motors have long been used in motor vehicles and trains. The reasons for this are:
- high efficiency (especially with partial load operation, important for battery operation),
- Uninterrupted torque output over the full speed range, no start-up synchronization or switchable gear ratio necessary. This means a high level of driving comfort (also important for electric wheelchairs, for example).
- Smaller size and mass than comparable internal combustion engine; This allows space-saving installation close to the wheels.
- No emissions ; therefore use in emission-sensitive areas possible (factory halls, tunnel areas and residential areas, for example)
- Lower operating costs (very long engine life, less maintenance).
- Simple structure including a simpler cooling system.
- Installation of an electromotive brake that enables regenerative braking with energy recovery and does not require any maintenance work due to wear , as is the case with conventional brake systems.
Despite these advantages, the electric motor has so far not been used much in cars and trucks. The reason is in particular the limited maximum range or the high mass of the energy storage ( accumulators ) and their long charging time.
Some model airplanes ( electric flight ), small ships, torpedoes and submarines are powered by an electric motor and an accumulator . The electric motors of other submarines are fed from fuel cells or from a small nuclear power plant that is carried along .
Vehicle drive concepts with electric motors, but with no or only partial energy storage in an accumulator, are:
- Fuel cell drive: One project is, for example, HyFLEET: CUTE, the continuation of the CUTE project . Problems exist in the life and cost of the fuel cells.
- Hybrid drive ( e.g. Toyota Prius ): A gasoline engine is combined with an electric motor / generator and buffered with accumulators (advantages in the partial load range / city operation, high driving comfort, regenerative braking ( recuperation ), buffering also with double-layer capacitors ).
- Gyro drive : a is used as the energy storage flywheel with a generator driving the driving motors (is, inter alia, in Gyrobussen applied, low range, regenerative braking is possible).
In the case of electric railways and trolleybuses , the electrical energy is supplied via overhead lines or busbars . Regenerative braking can also take place here if the supply network is designed for this or if accumulators are installed. Double-layer capacitors are also used here.
Another mobile application is the diesel-electric drive ; Here, a diesel generator generates electricity that drives the traction motors. Regenerative braking is not possible unless batteries are also carried. Diesel-electric drives can be found in ships, locomotives and submarines (here supplemented by an accumulator).
Applications in industry
The diverse areas of application of electric motors in industry can be divided into twelve areas. The first four deal with the material flow. The next four with continuous or clocked production lines and the last two with processes that affect the workpieces.
- Conveyor drives: The drives are required to have a long service life, robustness and reliability, low maintenance requirements, high modularity and low energy consumption. The drives are mostly used in continuous operation, so accelerations play a subordinate role.
- Travel drives are used on vehicles for material transport, for example in driverless transport systems , gantry cranes , storage and retrieval units or electric monorail systems . A high precision of the drives is required for the exact approach to positions.
- Hoist drives should convey goods vertically upwards. These include cranes , lifting tables , freight elevators and construction hoists .
- Positioning drives are used to move individual goods from one point to another. This includes the assembly of electronic components, the feeding and removal of workpieces in production machines and assembly machines. The majority of these drives are designed as linear direct drives.
- Coordinated drives for robots: Industrial robots often have up to six axes which should have a certain target position at the same time during movement. Coordination of the individual drives is therefore necessary in order to generate the desired movement of the robot arm.
- Synchronous drives are used in production processes in which a continuous product is manufactured as an endless material. These include transporting , rolling , coating , spinning and twisting as well as printing .
- Winding drives are often at the beginning or end of a continuous flow production. They are used, for example, in steel works to wind sheet metal into coils and in mechanical engineering and the automotive industry to unwind them again. Further examples are the winding and unwinding of wire, thread or paper. Since the wound material has an ever larger circumference over time, the circumferential speed increases while the speed remains the same. In order to prevent the product from tearing, the drives must be regulated over the circumference of the wound material.
- Cycle drives for cross cutters and flying saws are used in continuous production to separate the flow material, for example by sawing off a section. Special requirements result from the fact that the material moves on during the cutting process.
- Drives for electronic cams are drives with non-uniform movement. A punching tool, for example, should lower slowly in order to achieve good quality work on the workpiece and raise it quickly. Other applications are gluing , welding , bending and cutting.
- Drives for forming processes: These include the pressing of sheet steel, the extrusion of plastics, deep drawing or drop forging .
- Main and tool drives in machine tools . They are used to drive milling machines , drills and lathes . This is the only industrial use case for which there is extensive engineering literature.
- Drives for pumps and fans .
Efficiency and efficiency
Technologically outdated electric motors lead to increased energy consumption. In 1998 a voluntary agreement was reached between the European Sector Committee for Electric Drives CEMEP and the European Commission. In this now outdated agreement, three efficiency classes were defined:
- EFF3 = motors with low efficiency
- EFF2 = motors with improved efficiency
- EFF1 = motors with increased efficiency
In 2009, a new global standard for the efficiency classes (EN 60034-30: 2009) was introduced. The following efficiency classes for low-voltage three-phase asynchronous motors in the power range from 0.75 kW to 375 kW are applicable today:
- IE1 = standard efficiency (comparable to EFF2, sales since June 2011 only permitted to a limited extent)
- IE2 = high efficiency (comparable to EFF1)
- IE3 = premium efficiency
- IE4 = Super Premium (> 97% realized)
Since June 16, 2011, uncontrolled motors (0.75–375 kW) may only be placed on the market from power class IE2. The proportion of highly efficient engines is to be steadily expanded. Examples are permanent magnet synchronous motors with the highest levels of efficiency.
Manufacture of electric motors
The individual components of the electric motor are manufactured independently of one another. The most important are the housing, the stator, the shaft and the rotor. The final assembly then takes place.
Manufacturing of the housing
The actual housing is closed on both sides by covers which when electric motor bearing shields are referred to as they are also to the bearings of the motor shaft by means of ball bearings are used. However, the individual process steps for the end shields and the housing are the same. Both are first roughly shaped by casting or extrusion , followed by fine machining with turning , drilling and grinding and finally cleaning. The details depend on the number of items produced.
Casting with molds made of sand is only used for small quantities, for example in the production of prototypes. Die casting and centrifugal casting as well as extrusion are suitable for medium and large quantities . Die casting is the most common process with a share of 60%. Here the mold is made of steel and can be cast about 80,000 times. The machines required cost between 700,000 euros and one million euros, so that a minimum number of around 15,000 must be achieved in order to be economical. Centrifugal casting systems, on the other hand, only cost around 60,000 to 100,000 euros. The most expensive are extrusion presses with 8 million euros. They are therefore only suitable for very large series, but then have the lowest unit costs.
After casting or extrusion, the housing is deburred. Further fine machining is mostly done on machining centers that specialize in turning, drilling, milling and grinding. The tasks include boring out the inner contour, finishing the edges and drilling holes or threads.
In the case of small series, the housing is usually cleaned by blasting with dry ice (so-called dry ice blasting ) or with small balls ( shot blasting ). This removes casting residues, chips , dust and other dirt particles. In the case of medium-sized series, cleaning is carried out using an ultrasonic bath . In the case of large series, continuous cleaning systems are used, which consist of a feed station, cleaning and rinsing zones, the drying zone and the transfer station.
Manufacture of the sheet metal packages
The actual power-generating components, i.e. the rotor and the stator, are assembled from laminated cores. Compared to the solid material construction, laminated cores have the advantage that they prevent eddy currents and thus increase efficiency. When assembling the sheets into packs, it is important to avoid short circuits . The individual sheets are therefore coated with an insulator . They are made from electrical steel. These are sheets made from a silicon-containing steel that has improved magnetic properties. Since its production is quite complex, it is bought by electric motor manufacturers. The sheet metal stacks are manufactured in several steps: cutting out the sheets, stacking, permanent joining (gluing, welding, etc.) and reworking.
For smaller series or prototypes, the sheet metal is cut using laser or water jet cutting. Punching is more economical for larger series. The sheets are then stacked. With punching , this can be done directly in the machine, while with the other processes, a further process step is necessary. There are numerous options for joining the sheet metal stacks. In mass production, tabs are often pressed down on individual metal sheets, in recesses in the layers below. This step is often integrated directly into the punching process. After stacking, the individual layers can also be welded together. This is economical with significantly lower quantities, but has the disadvantage that an electrically conductive connection is created, which favors the development of eddy currents. Since the weld seams can be made in places that are of little importance for the magnetic field, the disadvantages in terms of efficiency are small. Another option is to use baking varnish . Here, after punching, the individual sheets are coated with baking varnish and stacked and then baked in the oven. As a result, the layers are glued together on the one hand and also isolated on the other.
The last step can be post-processing, which increases the efficiency somewhat. This includes stress- relieving annealing , external turning , deburring and repainting. Since the increases in efficiency are small, this is mainly done with large engines.
Manufacture of the stator
With a share of 35% of the total costs, the stator is the most expensive component. This is due to the complex production and the expensive material. The individual process steps are isolating the components, winding the coils, processing the winding and impregnating.
Insulator paper is used between the laminated core and the windings of the coils in order to avoid voltage flashovers. The wire required for the coils is produced by means of wire drawing , then coated with an insulating varnish layer and then with a sliding layer that makes winding easier.
In coil winding technology, numerous methods and processes have been established for producing the coils. The most important are the linear, flyer and needle winding technology. The systems for coil winding cost between 150,000 euros for simple machines and up to 4 million euros for systems for large-scale production.
After the coils have been built into the stator, the ends of the wires are contacted and tested.
Manufacture of the shaft
The cost share of the wave is very low at only 5%. Production takes place in three steps: rough machining in a soft state, hardening and fine machining by grinding.
In the case of large numbers, the first shaping is usually done by forging , in particular by drop forging . Machining centers are used for small and medium-sized quantities, as for the manufacture of the housing. Conventional heat treatment methods are used for hardening, including induction hardening , case hardening and nitriding . In all cases, the final shape is then precisely created by hard turning or grinding.
Manufacture of the rotor
In the case of motors with permanent magnets, the production steps of magnetization , magnet assembly, shaft assembly and balancing are interchangeable, but different sequences each have their own advantages and disadvantages.
A rotor cage is used instead for asynchronous motors. Mostly it is manufactured by means of die casting. When building a prototype, it is also soldered together from bars and rings . High-quality cages are made of copper, which has a higher conductivity than aluminum but is also around four times more expensive and only melts at 1084 ° C. Aluminum alloys, on the other hand, melt at 600 ° C. Therefore, aluminum molds can be cast around 50,000 times, while molds for copper can only be poured 100 times. Usually the melt is poured directly into the rotor slots.
Because of the variety of different engine types and possible quantities, there are large bandwidths and variants for final assembly, from exclusively manual assembly to fully automatic assembly lines.
The next step is to assemble the sensors. With asynchronous motors this is a tachometer and with motors with permanent magnets it is a position meter ( incremental encoder ). They are also shrunk on, pressed in or glued. Temperature sensors are also installed.
The sensors and the individual phases are then contacted with the connector.
Then the end shields are equipped with ball bearings and attached to the housing. In the last step, the final inspection takes place, as well as the resistance , insulation , functional and high-voltage test as well as a test of the power electronics .
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